1. Field of the Invention
The present invention relates generally to an electric machine winding arrangement, and in particular to a winding of a rotational electric machine stator.
2. Background Art
A winding of an electric machine, and in particular a rotational electric machine such as an electric motor and/or generator, is generally formed by connecting the coils of a corresponding stator in a predetermined manner. Referring to FIG. 1(a), a schematic diagram is provided of a conventional rotational electric machine 10 generally referred to as an adjacent pole wound machine. The conventional machine 10 generally comprises a rotor 12, a stator 14, and a plurality of coils 16 (e.g., 16a-16h). In particular, the conventional machine 10 of FIG. 1(a) is an eight-pole electric machine 10 having eight coils 16 (i.e., C1-C8) corresponding to a phase (i.e., the phase voltage Vp) of the eight-pole machine 10. As illustrated, a first group of four adjacent coils 16 (e.g., C1-C4) are electrically coupled in series to form a first circuit 18 and the remaining coils 16 (i.e., a second group of four adjacent coils C8-C5) are electrically coupled in series to form a second circuit 20. The winding (i.e., phase winding) of the adjacent pole wound machine 10 is generally completed by coupling the first 18 and second 20 circuits in parallel. Accordingly, the coils 16 of the first 18 and second 20 circuits generally cooperate to generate a magnetic field for driving the rotor 12.
In an ideal electric machine, the air gap 22 between the rotor 12 and the stator 14 is uniform. However, such uniformity may be difficult to achieve in practice. As illustrated in FIG. 1(b), a common cause of non-uniform air gap 22 is the presence of eccentricity in the rotor 12. Such eccentricity may cause the first group of coils 16 (i.e., the coils 16 forming the first circuit 18) to face a smaller air gap 22 than the air gap 22 faced by the second group of coils 16 (i.e., coils forming the second circuit 20) or vice-versa. As a result, the magnetic flux in the air gap 22 faced by, for example, the first group is generally higher than the magnetic flux in the air gap 22 faced by the second group. Such an imbalance in the magnetic flux generally results in imbalanced forces at the stator 12 and rotor 14. The imbalanced forces, in turn, generally increase vibration and noise associated with operation of the machine 10.
Referring to FIG. 1(c), a schematic diagram is provided of a conventional attempt to address the problems associated with non-uniform air gap 22. As illustrated in FIG. 1(c), an electric machine 40, generally referred to as a skip-pole wound machine, comprises a rotor 12, a stator 14, and a plurality of coils 16 (e.g., 16a-16h). As illustrated, a first group of four non-adjacent coils 16 (e.g., C1, C3, C5 and C7) are electrically coupled in series to form a first circuit 18 and the remaining coils 16 (i.e., a second group of four non-adjacent coils C8, C6, C4 and C2) are electrically coupled in series to form a second circuit 20. The winding (i.e., phase winding) of the skip-pole wound machine 40 is generally completed by coupling the first 18 and second 20 circuits in parallel. As further illustrated in FIG. 1(c), the conventional skip-pole wound machine 40 is additionally characterized by a coil 16 corresponding to a phase voltage (e.g., C1 and/or C8) being adjacent to a coil 16 having a point at the lowest potential of a corresponding circuit (e.g., C2 and/or C7).
Because the conventional skip-pole wound machine 40 generally provides two circuits (i.e., 18 and 20), each having an equal number of coils 16 facing the large and small portions of the air gap 22, imbalance in the forces at the stator 12 and rotor 14 is generally decreased. Accordingly, noise and/or vibration associated with the non-uniform air-gap 22 are generally decreased.
While the conventional skip-pole wound machine 40 may reduce the vibration and/or noise associated with a non-uniform air-gap 22, such a skip-pole arrangement 40 generally increases the maximum possible voltage between two coils 16 at a wire crossing (i.e., point where a segment of a first coil 16 contacts a segment of a second coil 16). For example, assuming that the phase voltage (Vp) is split evenly across the coils 16 in each of the first 18 and second 20 circuits, the conventional adjacent pole wound machine 10 of FIG. 1(a) generally has a maximum wire crossing voltage of 0.5 Vp. For instance, if a wire at or near terminal A corresponding to C1 of FIG. 1(a) makes contact with a wire at or near terminal B corresponding to C2, the voltage across the wires at the wire crossing will be substantially equal to 0.5 Vp (i.e., Vp−0.5 Vp=0.5 Vp). Similar analysis shows that the wire crossing voltage between any two adjacent coils 16 of the adjacent pole wound machine 10 cannot exceed 0.5 Vp.
In contrast, and still assuming that the phase voltage (Vp) is split evenly across the coils 16 in each of the first 18 and second 20 circuits, the conventional split-pole wound machine 40 of FIG. 1(c) generally has a maximum possible wire crossing voltage of Vp. For example, if a wire at or near terminal A corresponding to C1 of FIG. 1(c) makes contact with a wire at or near terminal B corresponding to C2, the voltage across the wires at the wire crossing will be substantially equal to Vp (i.e., Vp−0=Vp). Accordingly, the wire crossing voltage between two adjacent coils 16 (e.g., C1 and C2) of the split-pole wound machine 40 may equal Vp. Such an increase in the maximum possible wire crossing voltage is generally undesirable as it may produce premature failure of the machine 40.
Accordingly, it may be desirable to have an electric machine having a coil arrangement that reduces the effect of non-uniform air gap while also reducing the maximum possible wire crossing voltage between two adjacent coils of the machine.